If specifications relating to a DRAM are changed or the specifications have not been fully decided, there are cases where the supply voltage supplied from the outside in accordance with a request from the user differs. A supply voltage supply circuit for driving a word line is one circuit for which different supply voltages are required of externally applied power. Designing such a supply voltage supply circuit involves preparing a plurality of types of transistors having oxide-film thicknesses for which circuit operation is optimum, and designing the circuit based upon the plurality of types of transistors. If a booster circuit is designed assuming a case where the power supplied will differ depending upon the specifications, the problem of oxide-film withstand voltage and a current-consumption problem ascribable to gate leakage must be taken into account. If, in order to avoid these problems, a transistor for thick-film capacitance is used in the circuit in accordance with the higher voltage among voltages that have been set depending upon specifications, there is the danger that this will result in a booster circuit of larger area.
Patent Document 1 describes a supply voltage generating circuit in which capacitors having different film thicknesses are used selectively in dependence upon the operating supply voltage, thereby reducing circuit layout area and generating a stable supply voltage. This supply voltage generating circuit is one that boosts the operating supply voltage of the device and outputs the boosted voltage and has a first capacitor made of a thick insulating film, a second capacitor made of a thin gate insulating film, and a switch circuit for changing the circuit connections of the first and second capacitors in accordance with the operating supply voltage.
[Patent Document 1]
Japanese patent Kokai Publication No. JP-P2005-158098A